LONDON, Aug 6 (Reuters) - The first scientific analysis ofprobable human-to-human transmission of a deadly new strain ofbird flu that emerged in China this year gives the strongestevidence yet that the H7N9 virus can pass between people,scientists said on Wednesday.

Research published in the British Medical Journal (BMJ)analysing a family cluster of cases of H7N9 infection in easternChina found it was very likely the virus "transmitted directlyfrom the index patient (a 60-year-old man) to his daughter."

Experts commenting on the research said while it did notnecessarily mean H7N9 is any closer to becoming the next flupandemic, "it does provide a timely reminder of the need toremain extremely vigilant."

"The threat posed by H7N9 has by no means passed," JamesRudge and Richard Coker of the London School of Hygiene andTropical Medicine said in a commentary in the same journal.

The scientists who led the study stressed, however, that thevirus has not yet gained the ability to transmit from person toperson efficiently - meaning the risk is very low that it couldcause a human pandemic in its current form.

The new bird flu virus, which was unknown in humans untilFebruary, has so far infected at least 133 people in China andTaiwan, killing 43 of them, according to the latest World HealthOrganization (WHO) data.

Most cases have been in people who had visited live poultrymarkets or had close contact with live poultry in seven to 10days before falling ill.

The BMJ study, lead by Chang-jun Bao at the Jiangsu ProvinceCentre for Disease Control and Prevention, analysed a familycluster of two H7N9 patients - a father and daughter - inEastern China in March 2013.

The first "index" patient - a 60-year-old man - regularlywent to a live poultry market and fell ill five to six daysafter his last exposure to poultry.

He was admitted to hospital on 11 March. When his symptomsbecame worse, he was transferred to an intensive care unit (ICU)on March 15 but died of multi-organ failure on May 4, the studyreported.

The second patient, his healthy 32-year-old daughter, had noknown exposure to live poultry but provided direct bedside carefor her father in the hospital before he went to intensive care.

She developed symptoms six days after her last contact withher father and went into hospital on March 24. She was moved tothe ICU on March 28 and died of multi-organ failure on April 24.

Strains of the virus isolated from samples taken from eachpatient were "almost genetically identical" - a strongsuggestion that the virus was transmitted directly from fatherto daughter, the researchers said.

"To our best knowledge, this is the first report of probabletransmissibility of the novel virus person-to-person withdetailed epidemiological, clinical and virological data," theywrote.

Peter Horby of the Oxford University clinical research unitin Hanoi, Vietnam, who was not involved in this research, saidthe study raised the level of concern about H7N9 and reinforcedthe need for intensive surveillance.